Interference suppression system



y 20, 1941- w. A. SCHAPER 2,242,330

I INTERFERENCE SUPPRESSION SYSTEM v Filed D90. 11, 1939 INVENTOR W/LA/AM A. SCHAPHZ MU/f6 ATTORNEY Patented May 20, 1941 INTERFERENCE SUPPRESSION SYSTEM William A. Schaper, Cicero, Ill., assignor to Johnson Laboratories, Inc., Chicago, 111., a, cor- Duration of Illinois Application December 11, 1939, Serial No. 308,508

4 Claims.

This invention relates to high-frequency circuits and more particularly to those employed in radio receiving systems of the superheterodyne type, where the signal passes through one or more resonant circuits tuned to the signal frequency, is modulated with locally produced oscillations, then is demodulated and amplified at an intermediate frequency, and finally is demodulated again and rendered audible. The part of the system through which the signal passes before any change in frequency occurs usually includes an antenna circuit and one or two resonant circuits with or without a thermionic amplifying vacuum tube, and is commonly referred to as the preselector.

The adjacent-channel selectivity obtainable in a superheterodyne system is largely due to the efficiency of the intermediate-frequency amplifier which being operated at a fixed frequency,

may be designed to have a desired selectivity characteristic. The preselector of the receiving system serves to collect the desired signal, to aid in the rejection of undesired signals in adjacent channels, and to substantially reject signals of certain other frequencies to which a superheterodyne receiver tends to respond,

When the usual superheterodyne radio receiver is tuned to a desired signal, some response is likely to be obtained from a second signal which differs from the frequency of the desired signal by twice the intermediate frequency of the receiver. The undesired signal may be called an image-frequency signal. For example, if the desired signal has a frequency of 600 kilocycles and the intermediate frequency of the receiver is 465 kilocycles, a signal having a frequency of 600+(2X465) or 1530 kilocycles will be an imagefrequency signal. The ratio of the input voltage of image frequency required to produce a given output to the desired signal voltage required for the same output, is called the image-frequency ratio. A feature of the present invention is to substantially raise the value of this ratio by a novel arrangement of the preselector circuit components.

My invention is particularly intended for use in connection with resonant circuits which are tuned over a range of frequencies by inductance variation. Such a system is the one disclosed by Polydoroff in United States Patent No. 1,940,228,

in which a resonant circuit having an inductance coil and capacitor is adjusted over a range of frequencies by movement of a compressed comminuted ferromagnetic core relative to the inductance coil. This method of tuning is commonly called permeability tuning. An improved form of such a system is disclosed in my United States Patent No. 2,051,012. Both Polydoroffs original system and my improved system readily cover an adequate range of frequencies and may easily be ganged to provide multiple unit systems. Permeability tuning is especially adapted for use in the preselector of a superheterodyne radio receiver because its use permits the preselector to be designed so as to provide substantially uniform gain and selectivity throughout the frequency range, thus overcoming the chief cause of non-uniform performance in this type of receiver.

Although my invention may be advantageously employed in superheterodyne radio receivers having more than one resonant circuit which is variably tunable to the signal frequency ahead of the first demodulator, its efliciency is so high that excellent performance is realized from the use of a single such circuit in this portion of the receiver, thus providing a substantial reduction in the cost of manufacture.

One of the objects of my invention, therefore, is to provide a simple, inexpensive, and efiicient system of preselection. A further object of my invention is to substantially improve the imagefrequency ratio of a superheterodyne radio receiver, These and other objects are accomplished in the manner to be described below.

In accordance with my invention, image-frequency interference is cancelled out by causing opposing potentials to be developed across seriesconnected circuit elements in the control-electrode circuit of the first thermionic vacuum tube of the receiver. Since the preselector is tunable over a range of signal frequencies, the image-frequency cancellation means must respond over a different but properly related range of frequencies, and this must be accomplished automatically and simultaneously,

In the superheterodyne type of receiver, the intermediate frequency is usually lower than the lowest desired signal frequency, and the local oscillator frequency must differ from the resonant frequency of the preselector by a practically constant amount. The difference is equal to the intermediate frequency, the frequency of the oscillator usually being higher than that of themeselector, so that the oscillator must be tunable over a higher but narrower range of frequencies than that covered by the preselector. The image frequency, which in this case is higher than the signal frequency by twice the intermediate frequency, falls Within a range Which is still higher and narrower. For example, if a receiver has an intermediate frequency of 465 kilocycles and is intended for the reception of signals between 540 and 1600 kilocycles, the oscillator will be designed to cover a frequency range from 1005 to 2065 kilocycles, and the image-frequency cancellation arrangement will function over a range from 1470 to 2530 kilocycles. The corresponding ratios of maximum to minimum frequency are 2.96, 2.06 and 1.72, respectively. In accordance with my invention, movement of a single ferromagnetic core with respect to two coaxially disposed inductance coils tunes the preselector system to signals within an approximately threeto-one frequency range and simultaneously and automatically tunes an image-frequency cancellation means over a higher but narrower frequency range having a ratio of maximum to minimum frequency of approximately 60 percent that of the first-mentioned range.

The invention will be better understood if reference is'made to the accompanying drawing, in which:

Fig. 1 is a schematic diagram of the preselector portion of a radio receiver embodying a preferred form of my invention; and

Fig. 2 is an elevation, partly in section, of one form of coil system suitable for use in the preselector of Fig. 1.

Referring to Fig. 1, antenna I is connected to ground through resistor 2, and to grid 3 of vacuum tube 4 through inductor 5. Grid 3 is connected to ground through series-connected capacitors 6 and l. Antenna I is connected to the junction of capacitors 6 and I through inductor 8. Ferromagnetic core 9 is movable relatively to inductors and 8 to vary their respective inductance values and the inductive coupling between them. Cathode of vacuum tube 4 is grounded through bias resistor l l shunted by bypass capacitor l2. Vacuum tube 4 may function as an amplifier or otherwise, as for example as the first detector of a superheterodyne radio receiver.

As shown more clearly in Fig. 2 of the drawing, inductors 5 and 8 are mounted coaxially and core 9 is arranged to movably enter inductor 5. Inductors 5 and 8 are wound and connected in such a way that the magnetic fields created by the passage of signal current through them are in opposition. Y

In operation, the larger portion of the signalfrequency current flows through inductor 5, the system being adjusted for resonance at the highfrequency end of the tuning range by means of capacitor 6 with core 9 fully withdrawn. Inductor 8 tends to act as a primary winding due to its inductive coupling with inductor 5, so that the small signal-frequency current flowing through inductor 8 induces an additional voltage in inductor 5. Thus a relatively large voltage of signal frequency is developed across capacitors 6 and 1 in series and applied to the grid of vacuum tube 4. The larger portion of the image-frequency current, however, flows through inductor 8 which, with capacitor 1, forms a series circuit that resonates somewhat above the image frequency. This current produces a potential drop of image frequency across capacitor 1. The small portion of the image-frequency current which flows through inductor 5 and capacitor 6 produces a potential drop across capacitor 6.

Since the circuit comprising inductor 5 and capacitor 6 resonates at a frequency considerably lower than the image frequency, the imageacross capacitors B and 1 may be made substan' tially equal. Thus, since they are equal in amplitude and opposite in phase, substantially no image-frequency voltage is applied to grid 3 of vacuum tube 4.

Mathematically stated, the arrangement is such that cancellation of the undesired image signal occurs at a frequency where Cs :capacitance of capacitor 6 in micromicrofarads;

cqzcapacitance of capacitor 7 in micromicrofarads;

L5 :inductance of inductor 5 in microhenries;

Ls :inductance of inductor 8 in microhenries;

and

Lm mutual inductance between inductors 5 and 8 in microhenries.

In one successful embodiment of my invention in accordance with Figs. 1 and 2 of the drawing, resistor 2 has a Value of 10,000 ohms; inductor 5 Comprises a progressive universal winding of V single silk enamelled litzendraht wire 1.125 inches long on a form 0.221 inch in diameter, and has a 1000-cycle inductance of 158.4 microhenries; capacitor 6 has a value of 63 micromicrofarads; capacitor 1 has a Value of 272 micromicrofarads; inductor 8 of B. & S. 34 plain enamelled wire comprises five turns wound in the first 0.125 inch of axial length at the end the core enters followed by 63 turns for the next 1.0 inch on a form 0.425 inch in diameter, and has a total 1000-cycle inductance of 19 microhenries. Ferromagnetic core 9 has a diameter of 0.200 inch and a length of 1.25 inches. The mutual inductance between inductors 5 and 8 is 30.3 rnicrohenries. This embodiment covers a signalfrequency range from 532 to 1650 kilocycles, and provides an image-frequency ratio in excess of 1000 throughout this range. The signal-frequency voltage gain between antenna and grid is approximately 17 times. These performance figures are based upon the use of a standard 200- micromicrofarad dummy antenna.

It will be understood that various modifications may be made by those skilled in the art without departing from the scope of my invention, which is defined in the claims to follow below.

It will also be understood that where in the specification I state that one circuit is tunable over a narrower range of frequencies than a second circuit, reference is made to the fact that the ratio of the width of the band to the frequency at its center is less than in the case of the first circuit. Where it is stated in the specification that one circuit is tunable over a higher range of frequencies than a second circuit, reference is made to the fact that the minimum frequency of the first circuit is higher than the minimum frequency of the second circuit and that the maximum frequency of the first circuit is higher than the maximum frequency of the second circuit.

I have described my invention with particular reference to radio receivers of the superheterodyne type because this type is now in almost universal use, but it will be understood that my novel circuit arrangement may also be employed in radio receivers which do not employ the superheterodyne principle. Nothing in my circuit arrangement per se limits its use to receivers of the superheterodyne type. Thus, in any communication system, if there are interfering signals at some particular frequency or range of frequencies, my circuit arrangement may be employed to suppress these interfering signals, without any alteration of its operating principle, and without the necessity for any changes except those required to adapt the circuit constants to the frequencies to be received and to be suppressed.

Having thus described my invention, what I claim is:

1. An interference suppression system for a radio receiver having an antenna and a vacuum tube with a control electrode, including a first series resonant circuit the capacitive portion of which is in circuit between said control electrode and ground, a second series resonant circuit the capacitive portion of which is included in the capacitive circuit between said control electrode and ground, and means for simultaneously varying the inductance in said resonant circuits and the inductive coupling between said resonant circuits comprising a ferromagnetic core movable with respect to coaxially arranged inductive portion of said resonant circuits, whereby the s'ignal frequency voltage developed across the capacitive portion of said first resonant circuit is applied to said control electrode and the interference-frequency voltage across the same portion of said first resonant circuit is substantially cancelled out by the interference-frequency voltage which is developed across the capacitive portion of said second resonant circuit.

2. An interference suppression system for a radio receiver having an antenna and a vacuum tube with a control electrode, including a first inductance coil connected between said antenna and said control electrode, a pair of capacitors connected in series between said control electrode and ground, a second inductance coil coupled to said first inductance coil and connected between said antenna and the junction of said capacitors, and a ferromagnetic core movable with respect to said inductance coils, said inductance coils and said capacitors being so chosen that the entire,

signal-frequency voltage developed across said capacitors is applied to said control electrode and the interference-frequency voltage developed across said capacitors is substantially cancelled out by the interference-frequency voltage which is developed across one of said capacitors.

3. An image suppression system for a superheterodyne radio receiver having an antenna and a vacuum tube with a control elect-rode, including a first series resonant circuit the capacitive portion of which is in circuit between said control electrode and ground, a second series resonant circuit the capacitive portion of which is included in the capacitive circuit between said control electrode and ground, and means for simultaneously varying the inductance in said resonant circuits and the inductive coupling between said resonant circuits comprising a ferromagnetic core movable with respect to coaxially arranged inductive portions of said resonant circuits, whereby the signal-frequency voltage developed across the capacitive portion of said first resonant circuit is applied to said control electrode and the image-frequency voltage across the same portion of said first resonant circuit is substantially cancelled out by the image-frequency voltage which is developed across the capacitive portion of said second resonant circuit.

4. An image suppression system for a superheterodyne radio receiver having an antenna and a vacuum tube with a control electrode, including a first inductance coil connected between said antenna and said control electrode, a pair of capacitors connected in series between said control electrode and ground, a second inductance coil coupled to said first inductance coil and connected between said antenna and the junction of said capacitors, and a ferromagnetic core movable with respect to said inductance coils, said inductance coils and said capacitors being so chosen that the entire signal-frequency voltage developed across said capacitors is applied to said control electrode and the image-frequency voltage developed across said capacitors is substantially cancelled out by the image-frequency voltage which is developed across one of said capacitors.

WILLIAM A. SCHAPER.

CERTIFICATE OF CORRECTION.

Patent No. 2,2u2,55o. May 20, 19m.

WILLIAM A. SCHAPER.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows; Page 5, first column, line 57-58, for the word "portion" read -portions-; and that the said Letters Patent should be read vilith this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 1st day of July, A. D. l9LLl.

Henry Van Arsdale (Seal) Acting Commissioner of Patents.

CERTIFICATE OF CORRECTION Patent No. 2,2L 2,55o. May 20, 19m.

' WILLIAM A. SCHAPER.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: lage 5, first colux nn, line 57-58, for the wordfi ao rtion" read portions-; and that the said Letters Patent should be read v vith this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 1st day of July, A. D. 191d.

Henry Van Arsdale, (Seal) v Acting Gemmissioner of Patents. 

